Jawaharlal Nehru Technological University Hyderabad B.Tech II THERMAL ENGINEERING I Previous Question Paper (November/December - 2020 - R13)

In a two-stroke SI engine, the port timing diagram represents the opening and closing of the intake, transfer, and exhaust ports relative to the crank angle. Unlike the ideal diagram, which assumes instantaneous port opening and closing at Top Dead Center (TDC) and Bottom Dead Center (BDC), the actual diagram accounts for the finite time required for gas exchange.

The actual port timing diagram is characterized by the following features:

• Exhaust Port Opening (EPO): Occurs before BDC (typically 50° to 60° before BDC) to allow the high-pressure combustion gases to escape, reducing pressure before the transfer port opens.
• Transfer Port Opening (TPO): Occurs after EPO but before BDC (typically 30° to 45° before BDC). This delay ensures that the exhaust pressure has dropped sufficiently to prevent backflow of exhaust gases into the transfer passage.
• Transfer Port Closing (TPC): Occurs after BDC (typically 30° to 45° after BDC), allowing the fresh charge to continue entering the cylinder as the piston rises.
• Exhaust Port Closing (EPC): Occurs after TPC (typically 50° to 60° after BDC). This delay is critical to prevent the fresh charge from escaping through the exhaust port (short-circuiting) and to ensure the cylinder is sealed for compression.
• Inlet Port Opening/Closing: Controlled by the piston skirt, typically opening before TDC and closing after TDC to utilize the inertia of the incoming charge.

The comparison between the ideal and actual diagrams highlights the following differences:

The actual diagram is designed to optimize the scavenging process, where the fresh charge displaces the burnt gases. The timing is a compromise between maximizing the time for gas exchange and minimizing the loss of fresh fuel-air mixture through the exhaust port.

A high-performance ignition system in an internal combustion engine is critical for reliable operation and efficiency. The essential qualities of a good ignition system include:

1. Reliability and Durability: The system must operate consistently under varying engine speeds, loads, and temperatures without failure. Components like the ignition coil, spark plug, and contact breaker must withstand high thermal and mechanical stresses.
2. Sufficient Spark Energy: The system must provide enough energy to initiate combustion reliably, especially under high-pressure conditions or lean fuel-air mixtures, ensuring the spark is hot enough to overcome the quenching effect at the spark plug gap.
3. Correct Timing and Advance: It must deliver the spark at the precise moment required for optimal combustion. This includes an automatic spark advance mechanism to adjust timing based on engine speed and load, ensuring peak pressure occurs at the correct crank angle.
4. High Voltage Generation: The system must be capable of generating a sufficiently high voltage (typically 15,000 to 25,000 volts) to jump across the spark plug gap, even when the gap is fouled or the electrode is worn.
5. Maintenance Requirements: A good system should be relatively maintenance-free. Modern electronic ignition systems are preferred over mechanical contact-breaker systems because they eliminate wear and the need for frequent timing adjustments.
6. Compactness and Weight: The components should be lightweight and compact to facilitate easy installation and minimize the space required within the engine bay.
7. Efficiency and Low Power Consumption: The system should draw minimal power from the electrical system while maintaining high output, ensuring that the battery remains charged and the alternator is not unnecessarily taxed.
8. Resistance to Environmental Factors: The ignition system must be well-insulated and protected against moisture, oil, dust, and vibration to prevent short circuits or misfires.